1. Field of the Invention
The present invention relates to a sheet (such as paper, original) feed device which feeds a sheet placed on a sheet feed tray to a predetermined process position and, more particularly, to a sheet feed device including a detection sensor which detects the presence/absence of a sheet on a sheet feed tray and a sheet length detection sensor which detects the length of a sheet placed on the sheet feed tray by detecting the presence/absence of the sheet.
2. Description of the Related Art
Conventionally, a sheet feed device, which automatically feeds sheets placed on a sheet feed tray to a predetermined process position one by one, has been known. The sheet feed tray of this sheet feed device is provided with a detection sensor which detects the presence/absence of a sheet on the sheet feed tray, a plurality of detection sensors which are arranged along the sheet feed direction to detect the length of the sheet placed on the sheet feed tray by detecting the presence/absence of the sheet, and the like. As detection sensors provided on the sheet feed tray, a lever-type sensor detection scheme (lever-type sensor) or a reflection-type sensor detection scheme (reflection-type sensor) are known. The former scheme is designed to detect the presence/absence of a sheet when the sheet swings a detection lever provided to protrude from the sheet mount surface. The latter scheme is designed to cause light emitted from a light-emitting element to be reflected by a sheet and return to a light-receiving element, thereby detecting the presence/absence of a sheet in accordance with the amount of light returned from the light-emitting element to the light-receiving element.
The lever-type sensor, however, cannot detect a deformed sheet, such as a bent sheet or curled sheet, which is partly floating from the sheet mount surface. In contrast, the reflection-type sensor can detect a sheet even in a state wherein the sheet is slightly floating from the sheet mount surface. However, since the reflection-type sensor is provided on the sheet feed tray so as to face upward, the sensor is easily affected by disturbance light such as light from a fluorescent lamp.
As a sensor designed to solve the problem of disturbance light, a reflection-type sensor based on a pulse modulation scheme is available (see patent reference 1: Japanese Unexamined Patent Publication No. 58-156873). In this scheme, the light-emitting element of the reflection-type sensor is caused to emit light by a driving pulse, and the presence/absence of a sheet is detected by sampling the amount of light received by the light-receiving element at the timing synchronized with this driving pulse. There is also available a scheme of fixing a reflection-type sensor at an angle in a direction different from the direction in which external light directly strikes the light-receiving element of the sensor (see patent reference 2: Japanese Unexamined Utility Model Publication No. 62-96048).
Recently, however, an inverter fluorescent lamp which is pulse-driven at a high frequency is used in many cases. For this reason, in the former case wherein the reflection-type sensor is used, when light from the fluorescent lamp directly strikes the light-receiving element, the sensor malfunctions. In the latter case, since the reflection-type sensor is fixed in advance in the direction in which external light does not easily enter, if some limitations are imposed on the installation place of the sheet convey apparatus, there is a chance that disturbance light such as light from a fluorescent lamp cannot be effectively avoided, and a detection error may occur.